Canada serves the Formula One calendar as the first track where top speed is of overriding importance. Some of the other high-speed tracks have been modified (Hockenheim, for example) or now require higher amounts of downforce to cope with newly installed chicanes. This leaves only Montreal and Monza matching in their exceptionally high demand on the F1 cars.
The circuit Gilles Villeneuve is a semi-permanent facility and is formed largely of straights interconnected with chicanes and hairpins. There are no major corners to be taken at speed, and as a result the track doesn't require large amounts of downforce and rewards top speed.
For this race, teams aim to run the minimum downforce in order to maintain fast speeds on the two straights, leaving only downforce to cope with the heavy braking and to maintain some balance in the car's handling. To achieve this, teams used to develop a major upgrade for these low downforce races, but nowadays they are happier to revise the rear wing elements and front wing flap, both of which are flattened out to produce a lot less downforce.
Other aerodynamic devices on the cars are often removed, such as extra flip-ups and fins mounted around the car. This reduces the unwanted drag these items create. Furthermore, the weather can get very hot, which complicates the aero set-up, and for this race a lot of the teams have also added cooling vents in the top of the chassis to introduce a bit of cooling for the driver, who is sat amongst the power steering rack and electronic boxes.